1. Technical Field
This invention relates in general to semiconductor memories and, more particularly, to a method and apparatus for deskewing data signals from a semiconductor memory.
2. Description of the Related Art
DDR (dual data rate) memory was developed to increase the bandwidth of data transfers to and from a memory. DDR memory differs from SDR (single data rate) memory by transferring data at both the rising edge and the falling edge of a strobe signal. Accordingly, twice as much data may be transferred in a given time period.
The timing margins to capture the data transmitted from the memory to an accessing device (and from the accessing device to the DDR memory) can be seriously affected by an accumulation of different static and dynamic parasitic effects from source to destination points. The effects include:                transistor mismatches at the output of the transmitting device;        length mismatches between bonding wires at the transmitting device;        different lengths of the paths on the printed circuit board;        length mismatches between bonding wires at the receiving device; and        transistor mismatches at the output of the receiving device.        
Accordingly, the each bit of a received data word is likely to become stable at a slightly different time.
FIG. 1 is a timing diagram that illustrates the problem and the prior art resolution of the problem. A data word (DATA[7:0]) is received by an accessing device, and each data bit becomes stable within a time spread 10. The strobe signal transitions within the time spread 10. Hence, it may be possible that upon the transition of the strobe signal, some or all of the data bits have not become stable. To prevent errors due to unstable bit signals, a current solution adds a delay locked loop in the receiving device to generate a delayed strobe that is in an optimal timing window for data capture.
Nonetheless, the spread of data signals reduces the setup and hold margins. As the frequency of the memory accesses increases, the delayed strobe signal may be insufficient to ensure error-free operation.
Therefore, a need has arisen for a method and apparatus for accurately receiving multi-bit data from a memory at high speeds.